A woman tucks into a hamburger. The New York study found calorie labelling has an effect on only about one in six customers. Photograph: Getty Images/Amana images

Only a minority of people eating at fast-food outlets such as McDonald's or KFC are put off the more fattening options when the menu shows the calories of each meal, research in New York has found.

Some fast-food chains such as McDonald's, Pizza Hut, KFC and YO! Sushi have signed up to the UK government's public health responsibility deal on food, which includes a commitment to labelling their meals with the calories they contain from September. The UK deal is entirely voluntary. In New York, on the other hand, calorie labelling on the menus of fast-food restaurant chains has been compulsory since 2007.

But a study carried out at lunchtime in 11 fast-food chains before and after the New York legislation came in has found that calorie labelling does have an effect – although only on about one in six customers and only in certain food outlets. At McDonald's, KFC and the sandwich store Au Bon Pain, customers cut down on the calories. But overall, there was no reduction in the calorie intake of more than 8,000 customers, and at Subway the calorie intake actually went up by 17.8%.

Having healthier food on offer appeared to play a big part in the outcomes. Au Bon Pain, for instance, introduced a lower calorie Portions menu in 2008 while KFC added grilled chicken to the fried chicken on offer in April 2009, which cut the calories by 9%. Subway, on the other hand, added a supersized "$5 foot-long" sandwich in 2008 to appeal to the economically strapped, which three out of four customers in the study were buying by 2009.

The researchers, whose paper is published online in the British Medical Journal, say that customers buying lower calorie meals were often the same ones who said they were aware of the calorie information on the menu. Overall they bought food with around 100 fewer calories than the meals chosen by other customers.

Calorie labelling can be expected to have some effect on the fast-food chains, giving them an incentive to offer healthier foods, said the researchers. Starbucks switched from wholemilk to 2% fat as standard, for instance. "More changes in menu offerings may be expected as calorie labelling becomes effective nationwide." In a linked editorial, Dr Susan Jebb from the MRC Human Nutrition Research Centre in Cambridge says she believes that labelling is a step forward, but changes to the food on offer for us to buy must follow. "Calorie labelling will help consumers make an informed choice about what they eat, but sustained improvements in the nation's diet will require a transformation of the food supply too."

The British Heart Foundation called on fast-food chains to give their customers more information about the food they are selling: "Menu calorie counts are a great first step towards providing easily accessible information in restaurants about a whole range of nutrients, including saturated fat, sugar and salt.

"Fast-food retailers in the UK must take action to help diners make informed choices about the meals they buy."

Sign up for the Guardian Today

Our editors' picks for the day's top news and commentary delivered to your inbox each morning.